Straight from the horse's mouth:
FoundationDB is a distributed datastore, designed from the ground up to be deployed on clusters of commodity hardware.
The key-value store supports fully global, cross-row ACID transactions. That's the highest level of data consistency possible. What does this mean for you? Strong consistency makes your application code simpler, your data models more efficient, and your failure modes less surprising.
They say it's "actively developed and has years of production use". I wish they'd say exactly how it's being used in production. (Maybe they do and I haven't found it yet?) Also, if you're getting hung up on "key-value store", the vision is much bigger than that.